Geology Reference
In-Depth Information
EROSIONAL GLACIAL LANDFORMS
The 'knocks' are rocky knolls and the 'lochans' are lakes
that lie in depressions.
Glaciers and ice sheets are very effective agents of erosion.
Large areas of lowland, including the Laurentian Shield
of North America, bear the scars of past ice movements.
More spectacular are the effects of glacial erosion in
mountainous terrain, where ice carries material wrested
from bedrock to lower-lying regions (Colour Plate 12,
inserted between pages 208 and 209).
A panoply of landforms is moulded by glacial ero-
sion. One way of grouping these landforms is by the
dominant formative process: abrasion, abrasion and rock
fracture combined, rock crushing, and erosion by glacier
ice and frost shattering (Table 10.2). Notice that abraded
landforms are 'streamlined', landforms resulting from
the combined effects of abrasion and rock fracture are
partly streamlined, while the landforms resulting from
rock fracture are not streamlined. The remaining group
of landforms is residual, representing the ruins of an ele-
vated mass of bedrock after abrasion, fracturing by ice,
frost-shattering, and mass movements have operated.
Glacial troughs - glaciated valleys
and fjords
Glacial troughs
are dramatic landforms (Colour
Plate 13, inserted between pages 208 and 209;
Plate 10.4). They are either eroded by valley glaciers or
develop beneath ice sheets and ice caps where ice stream-
ing occurs. Most glacial troughs have, to varying degrees,
a U-shaped cross-section, and a very irregular long-
profile with short and steep sections alternating with long
and flat sections. The long, flat sections often contain
rock basins filled by lakes. In glacial troughs where a line
of basins holds lakes, the lakes are called
paternoster
lakes
after their likeness to beads on a string (a rosary).
The irregular long-profile appears to result from uneven
over-deepening by the ice, probably in response to vari-
ations in the resistance of bedrock rather than to any
peculiarities of glacier flow.
There are two kinds of glacial trough:
glaciated val-
leys
and
fjords
. A glaciated-valley floor lies above sea
level, while a fjord floor lies below sea level and is a
glaciated valley that has been drowned by the sea. In
most respects, glaciated valleys and fjords are similar land-
forms. Indeed, a glaciated valley may pass into a fjord.
Many fjords, and especially those in Norway, are deeper
in their inner reaches because ice action was greatest
there. In their outer reaches, where the fjord opens into
the sea, there is often a shallow sill or lip. The Sognefjord,
Norway, is 200 km long and has a maximum depth of
1,308 m. At its entrance, it is just 3 km wide and is 160 m
deep, and its excavation required the removal of about
2,000 km
3
of rock (Andersen and Borns 1994). Skelton
Inlet, Antarctica, is 1,933 m deep.
Breached watersheds and hanging valleys are of the
same order of size as glacial troughs, but perhaps generally
a little smaller.
Breached watersheds
occur where ice
from one glacier spills over to an adjacent one, eroding
the intervening col in the process. Indeed, the eroding
may deepen the col to such an extent that the glacier itself
is diverted.
Hanging valleys
are the vestiges of tributary
glaciers that were less effective at eroding bedrock than
the main trunk glacier, so that the tributary valley is cut
Abrasional landforms
Glacial abrasion produces a range of streamlined land-
forms that range in size from millimetres to thousands
of kilometres (Table 10.2). In sliding over obstacles, ice
tends to abrade the up-ice side or stoss-side and smooth it.
The down-ice side or leeside is subject to bedrock frac-
ture, the loosening and displacement of rock fragments,
and the entrainment of these fragments into the sliding
glacier base. In consequence, the downstream surfaces
tend to be rough and are described as plucked and
quarried.
Scoured regions
The largest abrasive feature is a low-amplitude but irregu-
lar relief produced by the
areal scouring
of large regions
such as broad portions of the Laurentian Shield, North
America. Scoured bedrock regions usually comprise a col-
lection of streamlined bedrock features, rock basins, and
stoss and lee forms (see below and Colour Plate 12). In
Scotland, parts of the north-west Highlands were scoured
in this way to give
'knock and lochan' topography
.
